<!DOCTYPE html>
<html lang="en">

<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
</head>

<body>
    <script>
        function balance() {
            let a = +prompt(`请输入银行卡余额`)
            let b = +prompt(`请输入当月食宿消费金额`)
            let c = +prompt(`请输入生活费消费金额`)
            return `当月银行卡余额为${a-b-c}`
        }
        document.write(balance())
    </script>
</body>

</html>